1. Field of the Invention
The present invention relates to a surface acoustic wave element having an electrode structure which is capable of representing high power resistance in a high frequency band and a method of manufacturing the same.
2. Description of the Related Art
Surface acoustic wave elements are electronic components using surface acoustic waves which are propagated in a state where mechanical vibration energy is intensively propagated only around surfaces of solid substances, and are used to construct filters, resonators, duplexers, etc.
Recently, a decrease in size and a decrease in weight of mobile communication terminals such as cellular phones have rapidly advanced, and thus a decrease in size of electronic components mounted on the mobile communication terminals has been required.
A surface acoustic wave element has a structure where a pair of interdigital transducer electrodes (IDT electrodes), which are made of a conductive material, are opposite to each other on a piezoelectric substrate and fingers thereof are alternately arranged. The surface acoustic wave element having such a simple structure is able to decrease the size of filters, resonators, duplexers or the like mounted on the mobile communication terminals.
A method of forming the interdigital transducer electrode portions of the surface acoustic wave element according to the related art will now be described. A first metal layer 4 made of an Al alloy, a second metal layer 5 made of Ti, and a third metal layer 6 made of an Al alloy, which adjusts the thickness of the electrodes, are sequentially formed on a substrate 1. Then, a pattern is formed through an ion milling method using Ar as a milling gas, and at this time, dispersion preventing layers 8 are formed on side walls of the electrodes, as shown in FIG. 10. The forming material of the dispersion preventing layers 8 is a mixture of the materials of the first metal layer 4, the second metal layer 5, and the third metal layer 6, that is, a mixture of an Al alloy and Ti. This surface acoustic wave element is disclosed in JP-A-2002-035702 (see pages 10 and 11 and FIGS. 21 and 22). In addition, as disclosed in JP-A-2002-035702, a protective film 9 made of silicon nitride is formed around the interdigital transducer electrode portions (electrode films), as shown in FIG. 11.
In addition, in a case in which the surface acoustic wave element is used as a resonator, it is important that variations of a serial resonant frequency and a parallel resonant frequency when the element temperature changes be reduced.
According to technologies disclosed in JP-A-7-15274 (see FIG. 1) and JP-A-8-265088 (see FIG. 1), the interdigital transducer electrodes and the piezoelectric substrate are covered with a silicon oxide film, so that variations of a serial resonant frequency and a parallel resonant frequency when the element temperature changes can be reduced.
However, if the interdigital transducer electrodes and the piezoelectric substrate are covered with a silicon oxide film, there is a problem in that voids are generated in the interdigital transducer electrode portions.
FIG. 12 is a plan view of a conventional surface acoustic wave element. The surface acoustic wave element has a structure in which an interdigital transducer electrode portion 13 and an interdigital transducer electrode portion 14 are formed on the piezoelectric substrate 12. The interdigital transducer electrodes 13 and 14 and the piezoelectric substrate 12 are covered with a silicon oxide film, which is not shown in FIG. 12. Connection electrode portions 15 and 16, which connect the surface acoustic wave element to external circuits, are electrically connected to the interdigital transducer electrode portions 13 and 14. FIG. 13 is a cross-sectional view of the interdigital transducer electrode portions 13 and 14, which are taken along one-dot chain line and viewed in the direction of the arrow. The section of the interdigital transducer electrode portions 13 and 14 shown in FIG. 13 is the same as that shown in FIG. 10, and the dispersion preventing layers 8 are formed at both sides. The interdigital transducer electrode portions 13 and 14 and the piezoelectric substrate 12 are covered with the silicon oxide film 10.
As shown in FIGS. 12 and 13, voids B are generated in the interdigital transducer electrode portions 13 and 14. In a heat treatment process in an atmosphere of oxygen and water when the silicon oxide film is formed on the interdigital transducer electrode portions 13 and 14 and the piezoelectric substrate 12, the forming materials of the interdigital transducer electrode portions 13 and 14 are dispersed in the insulating layer 10 and the oxygen and vapor are dispersed in the interdigital transducer electrode portions 13 and 14. As a result, the voids B are generated. In particular, when the interdigital transducer electrode portions 13 and 14 are formed of Cu or a Cu alloy, the voids are easily generated.
The conventional dispersion preventing layers 8 contain a mixture of an Al alloy and Ti, silicon nitride, or Ta. However, the dispersion preventing layers 8 formed of the above-mentioned materials cannot sufficiently prevent the oxygen and the water from dispersing into the interdigital transducer electrode portions 13 and 14.